The present invention relates to a method of driving a matrix display device comprising an electro-optical display medium between two supporting plates, an array of display elements arranged in rows and columns with each display element being constituted by electrodes provided on the facing surfaces of the supporting plates, and sets of row and column conductors, each display element being connected in series with a two-terminal non-linear switching device between associated row and column conductors, in which each row of display elements is selected during a row address period by a selection signal of a row signal waveform applied to the row conductors and data signals are applied via the column conductors for a part of the address period by means of which selection and data signals a range of operational voltages can be produced at the display elements for display purposes, a reference potential being applied to the column conductors during the remainder of the address period.
The invention also relates to a matrix display device which can be operated using such a method.
An active matrix display device of the above type is suitable for displaying video, for example, TV, pictures using passive electro-optical display media such as liquid crystal material, electrophoretic suspensions and electrochromic materials, although it can be used to display alphanumerical information instead.
A conventional drive scheme for liquid crystal display devices using two terminal non-linear switching devices involves applying a selection voltage to a row conductor during a selection period, corresponding in the case of TV display to a maximum of a TV line period, causing the switching devices associated with that row conductor to operate in the charging region of their characteristic so that the capacitances of the display elements concerned rapidly charge to a display voltage according to a data signal voltage present on the column conductor at that time. The display voltage produced lies in a predetermined operational voltage range used for picture display which in the case of a liquid crystal display device has lower and upper limits in the range of transition in the transmission/voltage characteristic of the liquid crystal material, commonly referred to as the threshold and saturation voltages respectively, at which the display element exhibits substantially maximum and minimum transmission, or vice versea depending on the relative orientation of associated polarising layers on the two supporting plates. At the end of the selection period the row conductor voltage falls to a lower, hold, value selected so that the mean voltage across the switching device during the period until it is next addressed, is minimised. Assuming an ideal situation, this hold voltage is equal to the mean of the rms saturation and threshold voltages of the display element.
In a known drive scheme, described in U.S. Pat. No. 4,892,389, the display element addressing period is shortened in order to reduce vertical cross-talk caused by the capacitance of the switching devices which can result in display element voltages varying over a field period due to data signal voltages intended for other display elements. This can be achieved by reducing the length of time for which data voltage is present on the column conductors to a fraction of the available line period, determined by the video signal. During the remainder of the line period, the column voltage is returned to a constant reference voltage level. At the same time the row selection period is reduced so that it is no greater than the duration of the data signal.
However, a problem can arise with such a drive scheme particularly, but not exclusively, where the switching devices comprise MIMs. The reduction in the row selection period means that the same display element capacitance must be charged in a shorter time which, in turn, means that a larger selection voltage must be used. This results in an increase in the peak current density in the switching device which can, if large enough, lead to damage or even destruction of the switching device.